This invention relates to a cylindrical, elongated, vertical furnace for treating materials at high temperatures, preferably above 1300.degree. C. and at high pressures, preferably above 500 bar.
Furnaces of this type are utilized for isostatic hot pressing of powder bodies or for condensation of previously sintered powder bodies, in a furnace compartment defined by a pressure chamber. Such chamber must be maintained sufficiently cool, as by cooling the walls thereof, if it is to be maintained structurally sound. The furnace compartment must therefore be properly insulated from the walls of the pressure chamber. And, because of the high pressure used in treating the bodies, the pressure medium normally has a high density and a very high thermal capacity. Argon may be used as a suitable, easily available and economical pressure medium which has, in addition a very low viscosity at the density and temperature involved, only four to five times that of air at atmospheric pressure, and is therefore quite mobile. Because of the very high heat transport capacity of the pressure medium it is important for the function of the furnace that the insulation around the furnace compartment be made in such a manner as to assure the smallest possible amount of heat convection between the furnace compartment and the space between its insulation and the pressure chamber walls.
The furnace according to the invention is of the type which includes a cup-shaped insulated portion having a closed upper end. Such a furnace is disclosed in more detail in commonly owned U.S. Pat. No. 3,732,068, the disclosure of which being specifically incorporated herein by reference. In this furnace an insulating casing which surrounds a furnace chamber includes an inner sheath of metallic material, which material limits the operating temperature in dependence on the high temperature strength of the metal. Life of the furnace is therefore reduced with increasing working temperature. At temperatures exceeding approximately 1300.degree. C., it is desirable to provide an improved insulation means, especially at 1500.degree. C., in order to extend the operating life of the furnace.